Swift's Burst Alert Telescope (BAT)

Instrument Description

The Burst Alert Telescope (BAT) is a highly sensitive, large FOV
instrument designed to provide critical GRB triggers and 4-arcmin
positions. It is a coded aperture imaging instrument with a 1.4
steradian field-of-view (half coded). The energy range is 15-150 keV
for imaging with a non-coded response up to 500 keV. Within several
seconds of detecting a burst, the BAT calculates an initial position,
decides whether the burst merits a spacecraft slew and, if so, sends
the position to the spacecraft.

In order to study bursts with a variety of intensities,
durations, and temporal structures, the BAT must have a large dynamic
range and trigger capabilities. The BAT uses a two-dimensional coded
aperture mask and a large area solid state detector array to detect
weak bursts, and has a large FOV to detect a good fraction of bright
bursts. Since the BAT coded aperture FOV always includes the XRT and
UVOT fields-of-view, long duration gamma-ray emission from the burst
can be studied simultaneously with the X-ray and UV/optical
emission. The data from the BAT can also produce a sensitive hard
X-ray all-sky survey over the course of Swift's two year
mission. Below is a cut-away drawing of the BAT, and a table listing
the BAT's parameters. Further information on the BAT is given by
Barthelmy (2000; 2005).

BAT Table of Instrument Parameters

Property

Description

Aperture

Coded mask

Detecting Area

5200 cm2

Detector

CdZnTe

Detector Operation

Photon counting

Field of View

1.4 sr (partially-coded)

Detection Elements

256 modules of 128 elements

Detector Size

4 mm x 4 mm x 2mm

Telescope PSF

17 arcmin

Energy Range

15-150 keV

Technical Description

The BAT's 32,768 pieces of 4 x 4 x 2 mm CdZnTe (CZT) form a 1.2 x
0.6 m sensitive area in the detector plane. Groups of 128 detector
elements are assembled into 8 x 16 arrays, each connected to
128-channel readout Application Specific Integrated Circuits
(ASICs). Detector modules, each containing two such arrays, are
further grouped by eights into blocks. This hierarchical structure,
along with the forgiving nature of the coded aperture technique, means
that the BAT can tolerate the loss of individual pixels, individual
detector modules, and even whole blocks without losing the ability to
detect bursts and determine locations. The CZT array has a nominal
operating temperature of 20 degrees C, and its thermal gradients
(temporal and spatial) are kept to within ±1° C. The
typical bias voltage is -200 V, with a maximum of -300 V.

The BAT has a D-shaped coded aperture mask, made of ∼54,000
lead tiles (5 x 5 x 1 mm) mounted on a 5 cm thick composite honeycomb
panel, which is mounted by composite fiber struts 1 meter above the
detector plane. Because the large FOV requires the aperture to be
much larger than the detector plane and the detector plane is not
uniform due to gaps between the detector modules, the BAT
coded-aperture uses a completely random, 50% open-50% closed pattern,
rather than the commonly used Uniformly Redundant Array pattern. The
mask has an area of 2.7 m2, yielding a half-coded FOV of
100 degrees x 60 degrees, or 1.4 steradians. A graded-Z fringe shield,
located both under the detector plane and surrounding the mask and
detector plane, reduces background from the isotropic cosmic diffuse
flux and the anisotropic Earth albedo flux by ∼95%. The shield is
composed of layers of Pb, Ta, Sn, and Cu, which are thicker nearest
the detector plane and thinner near the mask.

A figure-of-merit (FOM) algorithm resides within the BAT flight
software and decides if a burst detected by the BAT is worth
requesting a slew maneuver by the spacecraft. If the new burst has
more "merit" than the pre-programmed observations, a slew
request is sent to the spacecraft. Uploaded, rapid-reaction positions
are processed exactly the same as events discovered by the BAT. The
FOM is implemented entirely in software and can be changed either by
adjusting the parameters of the current criteria or by adding new
criteria.

BAT Operations

The BAT runs in two modes: burst mode, which produces burst
positions, and survey mode, which produces hard X-ray survey
data. In the survey mode the instrument collects count-rate data in
five-minute time bins for 80 energy intervals. When a burst occurs it
switches into a photon-by-photon mode with a ring-buffer to save
pre-burst information.

Burst Detection

The burst trigger algorithm looks for excesses in the detector
count rate above expected background and constant sources. It is
based on algorithms developed for the HETE-2 GRB observatory,
upgraded based on HETE-2 experience. The algorithm
continuously applies a large number of criteria that specify the
pre-burst background intervals, the order of the extrapolation of the
background rate, the duration of the burst emission test interval, the
region of the detector plane illuminated, and the energy range. The
BAT processor continuously tracks hundreds of these criteria sets
simultaneously. The table of criteria can be adjusted. The burst
trigger threshold is commandable, ranging from 4 to 11 sigma above
background noise with a typical value of 8 sigma. A key feature of
the BAT instrument for burst detection is its imaging capability.
Following the burst trigger, the on-board software checks for and
requires that the trigger corresponds to a point source, thereby
eliminating many sources of background such as magnetospheric particle
events and flickering in bright galactic sources. Time-stamping of
events within the BAT has a relative accuracy of 100 microsec and an
absolute accuracy from the spacecraft clock of ∼200 microsec.
When a burst is detected, the sky location and intensity are
immediately sent to the ground and distributed to the community
through the Gamma-Ray Burst Coordinates Network (GCN) (Barthelmy et
al. 2000).

Above is an example of the response of the BAT to a simulated
GRB. This shows a moderately difficult case: a GRB near the BATSE
threshold (0.3 count s-1 cm-2 in the 50-300 keV
band) in crowded field (point sources and diffuse+internal background
total 25x and 10x GRB count rate, respectively). When the count rate
in the entire detector plane increases by a significant amount, the
background-subtracted count rates in the individual detectors are
processed by a fast (but low resolution) algorithm to produce an image
of the entire FOV. The region of this coarse image containing the
brightest source (i, top middle) is selected for detailed
imaging. This high-resolution imaging uses an algorithm that is slower
but more accurate than the full-field algorithm. The image that
results (ii, top right) gives an accurate location for the GRB.

If there is no background subtraction, the resulting image will
contain bright steady sources that can be confused with the
transient. Images iii and iv (bottom middle and right)
show that, in this simulation, the GRB is still detectable in the
coarse and fine images, even though the steady sources are much
brighter.

Hard X-ray Survey

While searching for bursts, the BAT performs an all-sky hard
X-ray survey and monitors for hard X-ray transients.
The BAT accumulates detector plane maps every five minutes, which are
included in the normal spacecraft telemetry stream. Sky images are
searched to detect and position sources. The 5-sigma sensitivity of
the survey is about 0.8 mCrab in the 15-150 keV band for 2 years. For
regions where there are perpetually numerous strong sources in the BAT
FOV (i.e. the Galactic Center), the limiting sensitivity will be about
50% greater.

For on-board transient detection, one-minute and five-minute
detector plane count-rate maps and ∼30-minute long average maps
are accumulated in four energy bandpasses. Sources found in these
images are compared against an on-board catalog of sources. Those
sources either not listed in the catalog or showing large variability
are deemed transients. A subclass of long, smooth GRBs that are not
detected by the burst trigger algorithm may be detected with this
process. All hard X-ray transients are distributed to the
world community through the internet, just like the bursts.

Detector Performance

A typical spectrum of the 60 keV gamma-ray line from an
241Am radioactive source for an individual pixel is shown
in the figure below. It has a full-width-half maximum (FWHM) at 60
keV of 3.3 keV (dE/E = 5%), which is typical of CZT detectors. The
actual threshold varies between about 10 keV and 18 keV from detector
to detector. The average BAT background event rate is 10,000 events
s-1 (or about 0.3 count s-1 per detector), with
orbital variations of a factor of two around this value. This yields
a GRB fluence sensitivity of ∼10-8 erg cm-2
s-1 (15-150 keV).